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The paper describes a model of the soil system. There are two different, partly complementary means of building such a model. One approach is to construct, using empirical physical science methods, an isomorphic model, in which each and every component (variable) of the system is an element in the model. The other approach is to build, using a systems synthesis methodology, a homomorphic model, in which several components of the system are grouped to form a single element in the model. The latter method is adopted in this paper because of its suitability for modelling at the level of the whole soil system. As regards forming the basis of a homorphic model of the whole soil system, existing concepts of soil genesis have limitations: either they are very complex isomorphic models, the implementation of which presents virtually insurmountable problems; or they are homomorphs but too general to be used in their present form. And the concepts make no reference to the functional boundaries of soil system units. Indeed, such a unit has not yet been satisfactorily defined; the nearest to it is the soil catena, which is considered at length in the paper. Some of these problems are resolved by the concept of soil landscape systems which, synoptically, is as follows. A basic functional of the soil system, it is proposed, is a three-dimensional body of soil known as a soil landscape system or a “valley basin”, that (1) is bounded by the soil surface, valley watershed and weathering front at the base of the soil; (2) forms part of a more extensive valley basin network; and (3) functions as an open system. The flux of solids, colloids and solutes within and across the landscape is shown to be organized within the framework of these soil system units, and is discussed in terms of inputs, outputs and storages of the soil skeleton (clastic sediment) subsystem, the soil plasma subsystem and the soil solution subsystem. The value if the soil landscape system concept as a homomorphic model is stated. Five main advantages are outlined which relate to the five main features of the model: it has clearly and comprehensively defined basic system elements; it has functionally defined, three-dimensional system boundaries; the relationships between elements can be established from empirical hypotheses; there is transport and conservation of matter and energy as in the very successful models of hydrological systems and ecosystems; change of the system through time can be directly studied by computer simulation and not indirectly by statistical extrapolation from empirically derived behaviour patterns. The model, it is hoped, should provide a rational physical basis for simulating soil systems. |
